Malignant melanoma ranks second among adult cancers (behind adult leukemia) in potential years of life lost. Each year, over 47,000 new cases are diagnosed, and the incidence of cutaneous melanoma appears to be rising rapidly. Treatment of malignant melanoma involves surgical excision of the primary lesion, and vigilant monitoring to detect recurrence. Currently, there is no approved therapy for patients having intermediate risk of relapse. High-dose interferon, which can have serious side effects, is approved for treatment of patients having high-risk melanoma. There is no cure at this time for patients in whom metastasis to distant sites has occurred.
Dermatologists recommend that early detection of melanoma is the only way to reduce melanoma mortality by identifying curable lesions (Weinstock, JAMA 284:886-889, 2000). In practice, suspicious lesions are biopsied or excised, and examined by histology. Clinical suspicion, however, largely depends on the experience and the skills of the examining doctor. Very often the number of nevi under suspicion by far exceeds the number of lesions that could be removed. Erroneously excised lesions are costly for the public and cause scars often in cosmetically important body regions. A non-invasive molecular tool for diagnosis is therefore desirable. Furthermore, even after a lesion is taken out differentiation of benign melanocytic lesions from melanomas can be very difficult in a subset of about 10-15%, even for skilled dermatopathologists. If the diagnosis melanoma is made, tumor thickness is among the most important factors to roughly evaluate the prognosis of a melanoma patient (Breslow, Ann. Surg. 172:902-908, 1970). Molecular markers that could reflect the prognosis more precisely have also not been established. In addition, histological examination of cancer cells does not adequately reflect the complicated series of molecular events underlying the neoplastic process. Consequently, research efforts are now being focused on molecular profiling of cancer cells using DNA array technology, in which the activity of many genes or proteins are studied in parallel. Molecular profiling of human cancer has been recently reviewed (Liotta and Petricoin, Nature Reviews/Genetics 1:48-56, 2000).
Several approaches have been taken to simplify the analysis of gene expression profiles in tissue samples. Cell strains have been cultured to focus on a specific cell of interest. For example, some melanoma cell lines appear to accurately represent the original primary material (Bittner et al 2000). However, cultured cells lack the regulatory elements contributed by neighboring cells that affect gene expression in vivo, such as cell-cell communication molecules, soluble factors, and extracellular matrix molecules.
More direct methods for gene expression profiling of cellular subtypes include the global survey approach and microdissection. In the global approach, the information content of interacting cells is preserved by extracting RNA directly from a heterogeneous piece of tissue. To normalize the data set for the actual abundance of normal tissue, in relation to what is often minor amounts of diseased tissue, a reference gene set is constructed using RNA profiles extracted from a particular subtype of cultured cells. Preservation of mRNA in tissue samples in the clinical setting is challenging, since RNA is very labile and is susceptible to abundant tissue RNases (Liotta and Petricoin, Nature Reviews/Genetics 1:48-56, 2000).
An alternative approach, microdissection, utilizes mechanical force or laser capture methods to select a cellular subtype from a tissue sample, for subsequent molecular profiling. Transition stages from normal cells through carcinoma in situ, to invasive cancer can be identified microscopically, and profiled with microdissection techniques to discover molecular events occurring along the progression to malignancy. A disadvantage to this approach is the level of expertise and instrumentation required to select the pathological cells from the biopsied tissue sample.
Therefore, there remains a need for effective methods to detect malignant melanoma and to diagnose and stage melanoma from a suspicious skin lesion.